Estuche

So, I'm thinking of using an XTR M986 on a 10-speed commuter due to its clutch system. But the Shimano website says min/max 11t sprocket on the high and min/max 32t/36t sprocket on the low gearing.

What are the implications of deviating from this? For example, if I use a 12-25t Ultegra cassette and 10sp shifters (with Shiftmate), will the derailleur not be able to shift closer gear ratios at all?

tFUnK

Those are the limits. Any cassette within that range, including a 12-25 will work fine.

bikebikebike

The Rear Derailleur will have a limit on how much chain slack it can handle and how big a sprocket it can shift into without hitting something
A Front will have limits on how big a gap it can smoothly nudge a chain over. and how big a chainwheel it can pair up with.
One can often push things, but at the risk of rougher shifting and missed shifting and chain drop.
The factory designs for an optimized performance.
going smaller within the range is not usually an issue.
11 tooth is a usual boundary on any small freehub sprocket though rarely some have 12 as a limit (except Capreo which is a whole nother world).
Range (Hi to lo) is the parameter you watch for , smaller jumps between sprocket sizes is not a problem

Bill Kapaun

Put the RDER on the smallest cog & stand behind the bike.
Visualize the 2 different "ideal" paths the guide pulley would traverse going to the largest cog of 25 vs 32T.
It was that a "road" RDER was typically designed for the 12/13-23/27 range, while a "mountain" RDER was designed for an 11-30+ range. A much "steeper" angle.
That line is much more blurred now with 30T (or more?) 10 speed cogs and the introduction of >10 speed cassettes.
I run 12-23, 25 & 27T + 13-25T 9 speed cassettes on my Shimano "A" series mountain RDER's just fine.

base2 

Some of it pertains to the slope the parallelogram works on.

An 11-42 or 11-50 cassette was a very different slope of cogs than an 11-23 corncob.

The upper jockey wheel being a longer distance from the cog means that it must travel many more degrees (a longer distance) to effect a shift. Unfortunately that means that shifting will suffer terribly if you need 3 or 4 shifts worth of jockey wheel travel to go 2 or 3 cogs.

Similarly, a jockey wheel that is too close will not afford the chain the room it needs to go to smoothly be guided on to the next cog instead of driving into the side of it.


There are other reasons too. Some of them relate to cage length & chain uptake, but those are secondary to parallelogram slope.

One reason relates to spring tension & stress on the derailleur hanger. If the capacity of the derailleur cage is short & you put on a large cassette, your chain will be too short & there will be a lot of stress pulling every lever in the system tight like a drum. It's never pretty when failure happens.

Another reason relates to chain tension between long & short cage derailleurs. Short cage derailleurs hold the entire system under higher tension reducing chain slap at the expense of total capacity. Though clutches have largely mitigated this issue if you retrofit an IGH or other high friction freewheel mechanism the shorter lever (cage) you will appreciate the shorter cages heavy hand at keeping things in check. Short cages only work with more compact cassettes.

Retro Grouch 

""Application other than intended."

My personal philosophy is that, If I already own parts, I'll test assemble almost any combination of parts and see if I can make it work to my satisfaction. If I'm BUYING components, I'll only spend money for parts that are either in spec or that I know from previous experience will work together. I'm not too interested in other people's experience either. Some people are better mechanics than me and claim they can get things to work that leave me baffled. Some people have lower standards than I do. YMMV

jbucky1

Normally its just the max that matters. when selecting a derailleur and cassette.
It matters because the mechanics of the derailleur are designed to extend/ angle and work with a certain diameter (size) of cog therefore the larger the cog the different the implications on the mechanical design (pivots, lengths) of the derailleur

james
www.buckyrides.com

70sSanO

I have often wondered what the effect, if any, of running a smaller than minimum cog. I also wonder if establishing a minimum cog limit was just a marketing ploy in the presentation of the Dura Ace 7400 group. I mean was it true that a 6208 600 SIS rear derailleur couldn't run an 11t cog?

I guess the modern day equivalent is running a SRAM 10-42 cassette with a Shimano rear derailleur.

I imagine it really is just a marketing tactic.

John

jbucky1

I dont think its a marketing ploy. Having worked in these companies in the past, the engineering shines through. If they say it will only work with a 12t and not an 11t, theres a reason. It could be a huge reason or it could be a marginal reason but there is a technical reason that has fallen out of their limits / specification. Each company has different limits to this.

james
www.buckyrides.com